One apparatus used to measure the level of the fuel in a fuel tank, such as an aircraft fuel tank, includes a capacitive fuel probe. The capacitance of the fuel probe varies with the level of the fuel. In such designs, the capacitance of the fuel probe may be at a maximum value when the fuel tank is full, and at a minimum value when the fuel tank is empty.
Metal wires, such as wires formed from copper, are attached to the variable capacitor of the fuel probe and extend through the fuel tank, often along a substantial run (e.g., between about 15 feet and about 40 feet), to a location exterior to the tank. The low resistance wires terminate at a measurement circuit that uses the capacitance of the fuel probe, as measured at the termination of the wires, to provide an output signal that is a function of the capacitance of the fuel probe. The output signal of the measurement circuit may be part of or provided to, for example, a Fuel Quantity Indicating System of an aircraft, which generates fuel level data that may be displayed to a pilot, maintenance mechanic, or the like.
The accuracy of the measurement circuit depends on the transmission characteristics of the metal wires. In this regard, the capacitance of the fuel probe as measured at the termination of the run of metal wires is distorted by the resistance and capacitance of the wires. The total attenuation and inter-wire capacitance of the run increases with the distance between the fuel probe and the measuring circuit. However, long wire runs are increasingly specified in the design of large aircraft, where the total attenuation and inter-wire capacitance may limit accurate measurement of the fuel probe capacitance and, thus, the fuel level.